PARASITES OF LARGER BROWN-TAIL CATERPILLARS. 299 
ever was felt concerning the recovery of the parasite, which was 
considered to be as firmly established as the Calosoma or Compsilura 
concinnata. The results afforded another example of the obtrusive- 
ness of the unexpected. Not a single Parexorista puparium was 
secured from any of the material included in this series of collections. 
This was, all things: considered, the most serious setback of any 
which the parasite work has experienced since its inception. It was 
never doubted from the first that some among the parasites would 
be unable to exist in America, and no species was really credited with 
having demonstrated its ability to do so until it had lived over at 
least one complete year out of doors. Parexorista had done this 
and more, having gone through two complete generations, unless, 
what was not at all likely, its puparia had all been killed some time 
during the fall or winter. 
Without indulging in unnecessary speculation as to the reason for 
its disappearance, the following facts are presented for consideration: 
There is in America a tachinid known as Parexorista cheloniz, 
which is morphologically identical with the European race so far as 
may be determined through a painstaking comparison of the two. It 
is a common parasite of the tent caterpillars Malacosoma americana 
Fab. and MM. disstria Hiibn. The adult flies issue at the same time 
in the spring as do those of the European parasite of the brown-tail 
moth. The same type of egg is deposited; the larve are indistin- 
guishable in any of their stages or habits during their several stages; 
the third-stage larve issue at the same time and form puparia which 
are apparently the exact copies of the European, and the hibernating 
habits are the same. The one and only difference is that the Ameri- 
can Parexorista chelome does not attack the caterpillars of the brown- 
tail moth, while the European Parezorista chelonix is perhaps the 
most important of the tachinid parasites of this host. 
Mr. W. R. Thompson, whose excellent and painstaking work 
makes possible the above comparison between the two races, went 
a step further in his investigations. He found by actual experiment 
that in confinement, at least, the European males would unite with 
the American females with as much freedom as with those of their 
own species. Granted that similar intermingling of the races takes 
place in the open, and the reason for the nonrecovery of Parexorista 
cheloniz as a parasite of the brown-tail moth in the summer of 1910 
is no longer a mystery. 
It was stated a few paragraphs back that the flies which were colo- 
nized in the spring of 1908 were largely mated at the time of liberation. 
Their progeny, which issued in the spring of 1909, would therefore be 
of the pure-blooded European stock. Issuing at the same time were 
a vastly larger number of the American race, because as it happened 
there was an incipient outbreak of Malacosoma disstria in that very 
